1. Field of the Invention
The present invention relates to an improved process for the production of heavier hydrocarbons which combines autothermal reforming with air and the Fischer-Tropsch hydrocarbon synthesis reaction.
2. Description of the Prior Art
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is well known and is generally referred to as the Fischer-Tropsch reaction. Numerous catalysts have been used in carrying out the reaction, and at relatively low pressures (near atmospheric to 100 psig) and temperatures in the range of from about 300.degree. F. to 600.degree. F., both saturated and unsaturated hydrocarbons can be produced. The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity. The Fischer-Tropsch reaction can be characterized by the following general reaction: ##STR1##
Two basic methods have been employed heretofore for producing the synthesis gas utilized as feedstock in the Fischer-Tropsch reaction. They are steam reforming wherein one or more light hydrocarbons such as methane are reacted with steam over a catalyst to form carbon monoxide and hydrogen, and partial oxidation wherein one or more light hydrocarbons are combusted sub-stoichiometrically to produce synthesis gas.
The basic steam reforming reaction of methane is represented by the following formula: ##STR2## The steam reforming reaction is endothermic and a catalyst containing nickel is often utilized. The hydrogen to carbon monoxide ratio of the synthesis gas produced by steam reforming of methane is approximately 3:1.
Partial oxidation is the non-catalytic, sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The basic reaction is represented as follows: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2
The partial oxidation reaction is typically carried out using high purity oxygen which involves high costs. The hydrogen to carbon monoxide ratio of synthesis gas produced by the partial oxidation of methane is approximately 2:1.
A combination of partial oxidation and steam reforming, known as autothermal reforming, wherein air is used as a source of oxygen for the partial oxidation reaction has also been used for producing synthesis gas heretofore. For example, U.S. Pat. Nos. 2,552,308 and 2,686,195 disclose low pressure hydrocarbon synthesis processes wherein autothermal reforming with air is utilized to produce synthesis gas for the Fischer-Tropsch reaction. Autothermal reforming is a simple combination of partial oxidation and steam reforming where the exothermic heat of the partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby low cost is involved.
The autothermal process results in a lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, as stated above, the steam reforming reaction with methane results in a ratio of about 3:1 while the partial oxidation of methane results in a ratio of about 2:1. The optimum ratio for the hydrocarbon synthesis reaction carried out at low pressure over a cobalt catalyst is 2:1. When the feed to the autothermal reforming process is a mixture of light hydrocarbons such as a natural gas stream, some form of additional control is required to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio of about 2:1.
By the present invention, an improved process for the production of heavier hydrocarbons from one or more light hydrocarbons using a combination of autothermal reforming synthesis gas production with air and Fischer-Tropsch hydrocarbon synthesis is provided wherein the ratio of hydrogen to carbon monoxide in the synthesis gas is more efficiently controlled near the optimum ratio, and a nitrogen product stream is produced in addition to the heavier hydrocarbon product.